Israel's war against Hamas in Gaza has escalated into a "betrayal of humanity", the United Nations' humanitarian chief said Saturday.

In a statement on the eve of the six-month anniversary of the war, Martin Griffiths, the outgoing under-secretary-general for humanitarian affairs and emergency relief, called for a "collective determination that there be a reckoning for this betrayal of humanity."

"Each day, this war claims more civilian victims," said Griffiths, who will leave his post at the end of June due to health reasons.

"Every second that it continues sows the seeds of a future so deeply obscured by this relentless conflict."

The war began on October 7 with an unprecedented attack by Hamas militants resulting in the death of 1,170 people in southern Israel, mostly civilians, Israeli figures show.

Palestinian militants also took around 250 Israeli and foreign hostages, about 130 of whom remain in Gaza, including more than 30 the army says are dead.

Vowing to destroy Hamas, Israel has relentlessly bombarded the territory by air, land and sea, killing at least 33,137 people, mostly women and children, according to the health ministry in the Hamas-run territory.

Griffiths lamented "the unconscionable prospect of further escalation in Gaza, where no one is safe and there is nowhere safe to go."

He added that "an already fragile aid operation continues to be undermined by bombardments, insecurity and denials of access."

"On this day, my heart goes out to the families of those killed, injured or taken hostage, and to those who face the particular suffering of not knowing the plight of their loved ones," he said in the statement.